Bone formation and bone resorption are two highly related and balanced processes involved in normal bone remodeling. For osteoporosis patients, their osteoclast activity is elevated to such an extent that bone absorption rate exceeds bone formation rate. This process causes bone disease without the loss of bone mineral. Bone is also a frequent target for invasion and metastasis of tumors such as multiple myeloma, mammary cancer, prostatic cancer and lung cancer. During the metastasis process, cancer cells interact with the osteoclast precursors in bone marrow to differentiate into osteoclasts. Consequently, at the bone site for tumor metastasis, the number of osteoclasts is generally observed to increase, and the bone resorption caused by osteoclasts increases as well. In general, excess of osteoclasts, which differentiate from monocytes or macrophages is the major reason for osteoporosis and tumor related bone resorption. Osteoporosis often leads to bone fracture and happens quite often in old people, especially women. In China, according to the 5th population census in 2000, it is anticipated that in 2050 there will be 410 million people of 60 years old or above, about 27.4 percent of the total Chinese population. While the aged population increases rapidly, the susceptible population for osteoporosis quickly increases as well. For example, in the United States, there are 28 million osteoporosis patients, 1.5 million of which had bone fracture (700 thousand vertebra fracture and 250 thousand hipbone fracture). Therefore, WHO and the United Nations named 2000 to 2010 as “The Bone and Joint Decade”. According to the information provided by WHO, bone fracture caused by osteoporosis has increased 4 times in recent years, at the same time, bone diseases caused by the higher level osteoclast activity stimulated by tumor metastasis increases constantly. In the United States, bone resorption caused by tumor metastasis has led to about 500 thousand fractures, in which the multiple myeloma, mammary cancer, prostatic cancer and lung cancer are the most frequent sources. However, whether for women having the osteoporosis during menopause or patients having bone resorption caused by tumor metastasis, their diseases are both osteoporoses caused by the excess of osteoclasts. To solve the problem of osteoporosis, scientists all over the world are trying to find a cure to inhibit the excess of osteoclasts. In one case, Pfizer Inc. has submitted their application to the FDA for their new medicine Fablyn. Fablyn, a selective estrogen receptor modulator, is developed to cure osteoporosis of women during menopause by the similar mechanism to that of hormone estrogen. However, the application was declined twice in 2005 and 2006 because there were considerable chances of risk for the patients to have a carcinoma of the endometrium after the treatment. In 2007, Pfizer filed a new application for Fablyn as medicine for osteoporosis with new data. Amgen is also developing a protein medicine named Osteoprotegerin (OPG). OPG can potentially neutralize the activity of RANKL (Receptor Activator for Nuclear Factor KB ligand) to inhibit the excess of osteoclasts. In the meantime, researchers from Institute for Myeloma and Bone Cancer Research in Los Angeles are also working on a peptide that might reduce osteoclast formation: the tumor necrosis factor receptor-associated factor 6 (TRAF6) dominant negative binding peptide (TRAF6dn).
It is well known now that RANKL plays an important role in osteoclast formation. RANKL belongs to the TNF family. Its crystal structure indicates that a special cluster of its molecular structure can bind and activate the osteoclast precursor surface receptor RANK (Receptor Activator for Nuclear Factor κB). RANK is expressed on monocytes, chondrocyte and osteoclast precursor. It is the major receptor for regulation of osteoclast differentiation. RANKL/RANK has been proved the most important signaling pathway for osteoclast formation. During the tumor metastasis RANKL is the major physiological modulator for osteoclast differentiation. RANKL can be secreted directly by tumor cells, thus stimulates the osteoclast differentiation without the help of stromal cell.
Ever since the discovery of RNAKL/RANK signaling pathway, there has been rapid advance on understanding the regulation of osteoclast formation and activation. It has been found that, RANKL binds to the RANK receptor, which in turn phosphorylates ITAM tyrosine to activate DAP12 or FcRγ, as well as the co-activation pathway mediated by coreceptor TREM2 and OSCAR. The phosphorylation of tyrosine kinase ZAP70 and Syk leads to movement of PLCγ and Calcium ion, further activating NFATc1 which enhances osteoclast differentiation.
Osteoclasts are bone resorption cells formed by the fusion of monocyte-macrophage cells. Osteoclast formation requires RANKL and M-CSF co-stimulation as well as ITAM signal. However, human monocyte-macrophage also expresses an IgG inhibiting receptor FcγRIIb, which contains ITIM motif in its cytosolic domain. This receptor belongs to inhibitory receptor superfamily, it upregulates the cytosolic phosphatase SHP-1, while SHP-1 down regulates Syc-BCR to block the PI3K signaling pathway to down regulate ITAM and to inhibit osteoclast formation.
Cross-linking of FcγRIIb and ITAM motif containing receptor can inhibit ITAM's activation on calcium mobility and cell proliferation. Based on the balance of ITAM and ITIM in signaling pathway, the inventors have designed a fusion protein RANKL-Fcγ which can be used to cross link RANK and FcγRIIB receptor to induce ITIM signal thus to inhibit osteoclast formation and bone resorption.
As a cytokine, RANKL binds to the RANKL receptor of monocyte-macrophage cells to activate ITAM (Immunoreceptor tyrosine-based activation motif) signaling pathway. In this pathway, a series of different proteins are phosphorylated, resulting in osteoclast formation through the differentiation of monocyte-macrophages. In the meantime, ITAM signaling pathway is inhibited by ITIM (Immunoreceptor tyrosine-based inhibitory motif). ITIM counteracts ITAM signaling pathway through the activation of SHIP1 (SH2 domain-containing inositol polyphosphate 5′ phosphatase) and (SHPT/2) (SH2 domain-containing protein tyrosine phosphatase). In present invention, the inventors used genetic engineering and protein engineering technology to prepare a RANKL-Fcγ fusion protein, RIG, which inhibits the ITAM signaling pathway by activating cytosolic ITIM signaling pathway.
In summary, the first goal of this invention is to provide a DNA coding sequence for the fusion protein RIG that can be used to inhibit osteoclast formation.
The second goal of this invention is to provide fusion protein that can be used to inhibit osteoclast formation.
The third goal of this invention is to provide synthetic oligo-nucleotides used to prepare the above DNA coding sequence.
The fourth goal of this invention is to provide a preparation method for fusion protein RIG.
The fifth goal of this invention is to provide a medicine composition whose active ingredient is fusion protein RIG.